Lanthanide chromite ceramics generally comprise the family of compositions Ln1-xAExCr1-yByO3-δ, where Ln is a lanthanide element or yttrium, AE is an alkaline earth element, B is one or more transition metals, x<1 and y≦0.5) are technologically significant because they, among the lanthanide perovskite materials, maintain the greatest stability of crystal structure over oxygen partial pressures between 1 and 10−16 atmospheres. Over this range of oxygen partial pressures, the Cr cations do not change valence, which makes the crystal structure stable against the formation of oxygen vacancies and the concomitant increase in thermal expansion coefficient (“CTE”).
Intrinsic conductivity in the fundamental LaCrO3 structure is p-type (the primary charge carrier being electron holes) electronic conduction, which increases with temperature and the oxygen partial pressure. Extrinsic conductivity of LaCrO3 is increased by the addition of A-site alkaline earth dopants, which is compensated for on the B-site by Cr adopting 4+ valence in direct proportion to the alkaline earth doping concentration. This increases the number of electron hole charge carriers and the overall conductivity of the material in oxidizing and mildly reducing conditions. At strongly reducing conditions (PO2<10−8 atm), these doping strategies significantly increase the number of oxygen vacancies in the crystal structures. These doping strategies also increase the thermal expansion of the structure, an effect which is exacerbated at low PO2 by the increasing oxygen vacancy concentration.
The stability of lanthanum chromite materials makes them critical materials for components in electrochemical systems in which membranes are required to provide gas-tight, electrically conductive separation of oxidizing and reducing environments. Particular applications of interest include solid oxide fuel cells (“SOFCs”), where the ceramics provide the electrical interconnect between the anode and cathode layers of adjacent SOFCs in an SOFC stack. In electrochemical membrane systems, these materials can provide a similar interconnection function. Lanthanum chromite materials also can provide the electronic conduction path for composite mixed conducting (a two-phase mixture of oxygen and electronic conducting ceramic materials) membranes.
Additional applications of lanthanum chromite materials include their use as catalysts (combustion reactions for hydrocarbons and methane reforming), as well as anode materials for SOFC applications. These materials are also among those considered for sensors and other electrochemically driven devices.
For many of these applications, the lanthanum chromite material must be sintered to form a dense membrane. The sintering of LaCrO3 based ceramics is a notoriously difficult process. This obstacle alone has resulted in the failure of many SOFC programs to achieve manufacturability, as densification of the chromite requires sintering above 1600° C. to achieve targeted density values in the chemically pure material. Sr-(LSC) and Ca-(LCC) doping of the structure reduces the sintering temperature over the phase-pure LaCrO3 considerably, while raising its CTE. However, this approach still requires extremely high sintering temperatures to achieve dense parts.
The poor sinterability of the perovskite lanthanum chromite has been attributed to the formation of a volatile species, mainly CrO3, which then condenses as Cr2O3 at interparticle necks and prohibits the further densification. In order to suppress the CrO3 volatilization the materials were sintered at low pO2. Sintering can be improved by using a chromium deficient composition along with Sr or Ca doping. These dopants can react with chromium to form a liquid phase which aids sintering. Liquid formation is attributed to the melting of chromate species, SrCrO4 or CaCrO4, which exsolute from the perovskite. During sintering the chromate phases melt, enhancing densification, and then go back into solid solution in the perovskite, serving as a transient liquid phase sintering aid. The presence of the second phases acts to suppress the vaporization of the chromium component in LSC or LCC.
A range of processing approaches has been investigated in an attempt to overcome these obstacles. These approaches included the evaluation of SrVO4, SrCrO4 and CaCrO4 liquid phase formers, which create transient liquid phases during sintering. Sintering temperatures could be reduced to temperatures in the range of 1500-1600° C. The chromate liquid phases were studied within the concept of a transient liquid phase formation generated by A-site excess, which led to significant compromises in the resultant ceramic stability during atmospheric exposure to water. The parts effectively crumbled.
Additions of transition metals (Co, Cu, Ni, Fe, Mn, or V) to Sr-doped lanthanum chromite (“LSC”), particularly V, has been noted to reduce the sintering temperature needed for densification of lanthanum chromites. For example, a 5% addition of V to the B-site of the structure allowed densification of the material at 1500° C.
It has also been determined that ZnO additions to Sr-doped lanthanum chromite could reduce the sintering temperatures of the material. However, loss of ZnO at high temperatures (T>1550° C.) ultimately resulted in A-site excess, liquid phase formation and exaggerated grain growth.
Sr, Mn-doped lanthanum chromite (“LSCM”), which has been investigated for SOFC anode purposes, could provide adequate stability and performance as a ceramic interconnect material. This material offers a path to reduced processing temperatures compared to Sr-doped lanthanum chromite. For example, (La0.75Sr0.25)0.95(Cr0.5Mn0.5)O3 was co-sintered with zirconia and ceria interlayers at temperatures of 1500-1550° C.
There is a need for alternate methods for reducing the sintering temperature necessary for densification of lanthanide chromite ceramics that overcome the problems encountered with the prior art methods.